1. Field of the Invention
The present invention relates to a method of forming images such as multi-color images and, more particularly, to a multi-color forming method of superposing a plurality of toner images of different colors on an electrostatic photosensitive member by repeating a plurality of times an image forming process including at least charging, image exposing and toner developing steps.
2. Description of the Prior Art
There is known a color image forming method which uses an electrophotographic system so as to form electrostatic latent images in multi-color images.
According to this system of the prior art, the light from an original document has its colors separated through an optical filter, and the charging, exposing, developing and transferring steps are repeated for each of colors separated. Specifically, these steps are repeated three or four times so as to form images of respective colored particles (i.e., colored toners) of yellow, magenta and cyan colors, and a black color, if necessary. There is also the so-called "dichromatic developing method" by which electrostatic latent images of different polarities are formed on a common photosensitive member (i.e., image retainer) and developed with black- and red-colored particles. These multi-color image forming methods are desirable because they can add color information in addition to the information which is obtained from an image of white and black colors only, but are accompanied by the following problems:
(1) Since each image has to be transferred to transfer members each time development of each color is ended, the machine is large, and the considerable time is required to form the image. PA0 (2) It is necessary to guarantee the precision because of registration error due to repeated operation.
In view of these problems, attempts have been made to reduce the size of the machine by developing a plurality of toner images on a commo photosensitive member so that the transfer step may be conducted one time.
According to this image forming method, it is sufficient to repeat several times the subsequent developments of the photosensitive member which has already been formed with the toner images. However, this method is accompanied by a problem that the toner images formed in advance on the photosensitive member at preceding steps are disturbed during the succeeding development, or that the toners existing already on the photosensitive member are returned to a developer carrier, so that they are mixed into a succeeding developing device reserving a toner of a color different from those of the preceding developing agents thereby to invite somberness. In order to eliminate that problem, there has been proposed a method in which a photosensitive member and the developer layer on its carrier are held out of contact except in a developing device for first forming a toner image on the photosensitive member and in which an a.c. component is superposed on a developing bias to be applied to the developer carrier or the photosensitive member.
The principle of the aforementioned image forming method will be explained with reference to FIG. 1. This Figure is a flow chart showing changes in the potential on the photosensitive member.
First of all, the photosensitive member is charged to a constant potential by means of a scolotron charger o the like. The explanation will be continued in the following by taking up the case in which the charging polarity is positive. If the photosensitive member (as shown in FIG. 1 .circle.1 ) charged as above is exposed to an image, its optically irradiated part PH has its potential dropped (as shown in FIG. 1 .circle.2 ). "DA" indicates an area left unexposed. Next, by applying a bias having a d.c. component substantially equal to the potential of the unexposed part DA to a developing device in the first stage, the positively charged toner T.sub.1 in the developing device sticks to the exposed part PH having a relatively low potential so that a first visible image is formed (as shown in FIG. 1 .circle.3 ). As a result that the positively charged toner sticks, the corresponding part has its potential raised slightly (as indicated at DUP in the same Figure). Next, the photosensitive member is charged again uniformly by a charger, including the part having the toner T.sub.1 (as shown in FIG. 1 .circle.4 ). Next, a second image exposure is conducted (as shown in FIG. 1 .circle.5 ), followed by a development like the above. Then, another toner T.sub.2 sticks to the exposed part PH so that a second visible image is formed (as shown in FIG. 1 .circle.6 ). If these steps are repeated four times, for example, toner images of four colors are formed on a drum of photosensitive member. These toner images are transferred to a sheet of recording paper and then fixed to form a record. On the other hand, the surface of the photosensitive member is cleaned.
In the systems described above, the second and later charging steps can be omitted. If the charging steps are not omitted, a statically eliminating step may be incorporated before the charging steps.
In the explanation of the image forming method described above, there is taken up the case in which a reversing phenomenon of applying the toners to the exposed part of the photosensitive member is used. However, this image forming method can be practised even if the normal phenomenon of applying the toners to the unexposed part is used.
Here, an arbitrary color can be expressed on principle by the yellow, magenta and cyan (i.e., primaries) toners, and it is unnecessary to add a black toner. In the ordinary record, however, black is frequently emphasized more than other colors because they are used to express sharp parts such as letters or lines. Generally speaking, in order to express black in the primaries, moreover, it is required to strictly register the toner images of three kinds and to make the spectral transmissivities of the respective toners approximately ideal. However, these requirements are technically difficult to satisfy. Moreover, the density of the recorded image composed of the primaries is adversely affected. In order to eliminate those problems, therefore, the aforementioned image forming method is frequently practised by a developing device reserving the black toner in addition to those for the primaries.
The following two systems exist in case various colors are expressed by the methods described above:
(1) A system in which toners of different colors are not superposed directly one on the other; and
(2) The system in which toners of different colors are superposed one on the other.
According to the system (1), a color mixing is substantially caused on the recording paper by not superposing but distributing the multi-color toners T.sub.1 and T.sub.2 on a photosensitive member 1, as shown in FIG. 2(A). According to the system (2), toners of different colors are developed in a superposed manner on a toner image of a certain color by controlling a latent image potential and a developing bias.
In system (1), however, it is necessary to strictly register the image exposure so that the toner images of the respective colors may not be superposed in the common positions. If an image exposing ray L is so incomplete as is shown in FIG. 2(B), the toner image T.sub.1 of the preceding stage blocks the image exposing ray L partially to raise a problem that the applied amount of the toner T.sub.2 to be developed at the succeeding stage is so remarkably reduced as is shown in FIG. 2(C). This problem makes it impossible to establish color mixing of a desired tone. In the system (2), on the other hand, even if the toner T.sub.1 developed before is irradiated with a ray, this ray is absorbed by the toner T.sub.1 so that the latent image is not formed completely. As a result, the applied amount of the toner T.sub.2 developed later is so remarkably reduced as is shown in FIGS. 3 or 4.
As the multi-color image forming method described above, moreover, there are known methods which are disclosed in Japanese Patent Laid-Open Nos. 56-144452, 58-116553, and 58-116554. According to all these methods, the image exposures repeated are conducted by different apparatus so that the subsequent image exposure position has to be shifted not to be superposed on the previous image exposure position. Moreover, development is performed by applying such a toner to an electrostatic image corresponding to the exposure part and having a lower potential than the background potential so as to charge the same with the same polarity. If the image exposure is conducted to form a multi-color image such as a landscape image by means of a spot-distributed exposure using a laser beam scanner, for example. This raises the problem that the spot distribution density is made coarser to form only such a multi-color image as is seen rough and faded. Other problems are that the recording device is large thereby increasing manufacturing costs, and that the synchronous control of the exposure of the image retainer to the image is complicated because it is related to the position of the image exposure device.